Much has been described in the art about the various types of pacemakers. For example, reference is made to commonly assigned U.S. Pat. No. 4,712,555, to Thornander et al., where some helpful background information about pacemakers and the manner in which they interface with a patient's heart is presented. This patent is hereby incorporated by reference in its entirety.
In general, both single and dual-chamber pacemakers are classified by type according to a three letter code. In this code, the first letter identifies the chamber of the heart that is paced (i.e., the chamber where a stimulation pulse is delivered) with a "V" indicating the ventricle, an "A" indicating the atrium, and a "D" indicating both the atrium and ventricle. The second letter of the code identifies the chamber where cardiac activity is sensed, using the same letters to identify the atrium, ventricle, or both, and where an "0" indicates that no sensing takes place.
The third letter of the code identifies the action or response taken by the pacemaker. In general, three types of actions or responses are recognized: (1) an Inhibiting ("I") response, where a stimulation pulse is delivered to the designated chamber after a set period of time unless cardiac activity is sensed during that time, in which case the stimulation pulse is inhibited; (2) a Trigger ("T") response, where a stimulation pulse is delivered to the designated chamber of the heart a prescribed period after a sensed event; or (3) a Dual ("D") response, where both the Inhibiting mode and Trigger mode are evoked, inhibiting in one chamber of the heart and triggering in the other.
A fourth letter, "R" is sometimes added to the code to signify that the particular mode identified by the three letter code is rate-responsive, where the pacing rate can be adjusted automatically by the pacemaker, based on one or more physiological factors, such as blood oxygen level or the patient+s activity level. As used herein, "(R)", for example DDD(R), refers to the occurrence of two modes, such as DDD and/or DDDR.
Thus, for example, a DDI pacemaker is capable of sensing and pacing in both chambers, and operates in a non-atrial tracking mode. It inhibits ventricular stimulation pulses when a prior ventricular activity is sensed.
A DDDR pacemaker represents a fully automatic pacemaker which is capable of sensing and pacing in both the atrium and ventricle, and which is also capable of adjusting the pacing rate based on one or more physiological factors, such as the patient's activity level. In general, DDD(R) pacing has four functional states: (1) P-wave sensing, ventricular pacing; (2) atrial pacing, ventricular pacing; (3) P-wave sensing, R-wave sensing; and (4) atrial pacing, R-wave sensing. Advantageously, for the patient with complete or partial heart block, the P state of the DDD(R) pacemaker tracks the atrial rate which is set by the heart's SA node, and then paces in the ventricle at a rate that follows this atrial rate. Because the rate set by the SA node represents the rate at which the heart should beat in order to meet the physiologic demands of the body (at least for a heart having a properly functioning SA node) the rate maintained in the ventricle by such a pacemaker is truly physiologic.
Those skilled in the art have long recognized the advantages of using an atrial tracking pacemaker. For example, U.S. Pat. No. 4,624,260, to Baker, Jr. et al., discloses a microprocessor controlled dual-chamber pacemaker having conditional atrial tracking capability. Similarly, U.S. Pat. No. 4,485,818, to Leckrone et al., discloses a microprocessor-based pacemaker which may be programmed to operate in one of a plurality of possible operating modes, including an atrial rate tracking mode.
However, in some instances, a given patient can develop fast atrial rhythms which result from a pathologic arrhythmia such as a pathological tachycardia, fibrillation or flutter. In these cases, a DDD(R) pacemaker can pace the ventricle in response to the sensed atrial arrhythmia up to the programmed maximum tracking rate (MTR).
Sometimes it is possible at the time of implantation of a pacemaker to determine whether an atrial fibrillation, atrial flutter, or atrial tachycardia condition is going to develop. In such instances, the pacemaker can be programmed to operate in a different mode of operation, the leads can be repositioned within the heart, or other actions may be taken to minimize the likelihood of such pathologic arrhythmias occurring. However, it is not always possible at the time of implantation to determine whether a patient will develop an atrial arrhythmia after the pacemaker is implanted.
Therefore, if such pathologic arrhythmias subsequently occur, they must be treated using other techniques, such as through the administration of drugs. Needless to say, the administration of drugs normally requires the attendance of a physician. However, a physician is not always present when such pathologic arrhythmias develop, and even when a physician is available, such drugs also can undesirably suppress the ability of the SA node to increase the sinus rate during periods of exercise, emotional stress, or other physiologic stress. Thus, the use of such drugs can prevent the pacemaker from functioning as a intrinsic physiologic rate-responsive pacemaker.
As a result, attempts have been made in the art to prevent undesirable tracking of pathologic atrial arrhythmias by automatically switching the pacemaker's mode of operation from an atrial tracking pacing mode to a non-atrial tracking pacing mode. For example, U.S. Pat. No. 4,722,341, to Hedberg et al., describes an atrium-controlled pacemaker, where the pacemaker temporarily switches from an atrial tracking mode to a non-atrial tracking mode for a fixed number of stimulation pulses if the sensed atrial activity indicates an atrial arrhythmia may be developing.
U.S. Pat. No. 4,944,298, to Sholder, which is hereby incorporated by reference in its entirety, discloses an atrial tracking pacemaker with automatic mode switching capability. The pacemaker has the capability of setting a tachycardia rate limit (TRL) slightly above a maximum tracking rate (MTR), so that mode switching to a non-atrial tracking mode occurs when the TRL is exceeded. A third threshold rate is also set at a value below the MTR. The pacemaker switches back to an atrial tracking mode when the patient's atrial rate drops below this third threshold. To avoid mode switching based on a single short atrial interval between atrial events, the atrial rate is continuously averaged over several cycles. This technique effectively prevents frequent mode switches in patients whose atrial rates "hover" around the MTR.
U.S. Pat. No. 5,549,649, to Florio et al., which is hereby incorporated by reference in its entirety, describes an implantable dual-chamber pacemaker programmed to operate primarily in an atrial tracking mode. The pacemaker automatically switches its mode of operation from the atrial tracking mode to a non-atrial tracking mode in the event a filtered atrial rate exceeds a prescribed upper rate limit. This mode switching is accompanied by a corresponding switching from a primary set of operational parameter settings for the primary mode, to an alternate set of operational parameters for the alternate mode.
In general, however, atrial pacing competition could result during mode switching if the mode being switched to is a dual-chamber, non-atrial tracking mode (i.e., DDD, DDDR, VDD or VDDR), leading to atrial arrhythmia. Yet another concern associated with such mode switching is the occurrence of functional atrial undersensing that can lead to inappropriate atrial pacing. This undersensing is due to the long refractory periods in the DDI and DDIR modes, where pacing and sensing events are counted by the filter, leading to a measured filtered atrial rate that is higher than the actual atrial rate. In this condition, it is possible to delay or prevent the pacemaker from switching back to the atrial tracking mode.
Therefore, it would be desirable if the pacemaker could substantially reduce atrial pacing competition and atrial undersensing when switching from an atrial tracking mode to a non-atrial tracking mode, and to prevent mode switch lockup.